Sunspots & How They Affect Ionospheric Radio Propagation

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One of the features of the Sun that greatly affects HF ionospheric radio propagation is the dark spots or Sunspots that appear on the surface of the Sun.

These sunspots and their very approximately 11 year cycle play a major role in determining the state of the ionosphere because associated with them is an increase in the level or radiation emanating from the Sun, and hence the ionisation levels int he ionosphere.

First observations on sunspots

Sunspots have been observed for many years. The first observations were probably made by the ancient Chinese civilisation which observed and recognised them well before the birth of Christ.

However it was not until the early 17th Century after Galileo had invented the telescope that he was able to observe them more closely but projecting an image of the Sun onto a piece of paper. In fact several of his original sketches are still in existence today.

Even though the existence of sunspots was recognised in the 17th Century, it took some while before sunspot numbers were recorded.

In fact it was in the middle of the 18th Century that the first records of sunspot numbers were kept on a regular basis.

It was noticed that the numbers varied significantly and apparently in a random fashion. As the variations appeared random it took some years before a distinct cyclical form was noticed.

A scientist named Schwabe observed the Sun for almost 20 years and mathematically analysed the results and it was he who first discovered the cyclical variation of the sunspots, publishing his findings in 1843.

Today further research has revealed many more details about these spots, but there is still plenty more to discover.

What are sunspots?

It is not often that people set up a system whereby sunspots can be seen, but it is possible to project an image of the Sun onto some card to see the spots if there are any visible.

The sunspots appear as dark areas on the surface of the Sun that can be seen from time to time. The sunspots can last anything from a few hours right up to several weeks. The spots will also rotate with the rotation of the Sun.

The sunspots are spots are cool areas (relatively speaking) on the surface of the Sun or photosphere to be more exact. The temperature of the spots is around only 3000°C against a sizzling 6000°C for the rest of the surface.

When sunspots first appear they are first seen as a very small dark spot called a pore. Not all these pores develop into full spots, but the ones that do steadily develop over a period of hours or days to become full spots.As the pore becomes larger a lighter area is seen around the dark spot. The dark spot itself is referred to as the umbra and the lighter are around it is the penumbra.

It is found that sunspots often appear in clusters. Each spot can be anywhere between a few hundred miles in diameter to large ones which may be almost 100 000 miles across. The groups themselves can contain several large spots and may be over a quarter of a million miles in diameter.

The sunspots form out of the very intense magnetic fields that exist below the surface of the sun. These fields change during the course of the sunspot cycle.

It is found that at the solar spot minimum the magnetic fields are longitudinal running from the Sun's north to its south.

Although the Sun has a nominal rotation period of 27 days, different parts of the Sun rotate at different speeds as it is fundamentally a gaseous giant.

The equatorial regions spin more slowly than the poles and this causes distortion of the fields that slowly align in an east west direction. It is found that as a result of these changes the activity on the Sun changes in a cyclical form having a distinct trough and peak.

These changes in magnetic fields cause eruptions to take place through the Sun's surface. The magnetic fields can be incredibly strong. Around the eruptions the surface temperature falls dramatically giving rise to what are seen as dark spots - the sunspots.

The temperature around the sunspot produces a very large temperature differential, and there is also a large disturbance of the magnetic field in this region.

In fact the magnetic fields are very strong - often many thousands of times that of the Earth's magnetic field. When spread over the vast distances involved, the forces created by these magnetic fields is enormous.

Also, when a group of spots appears, as often happens, it is found that, one cluster will have a positive or north magnetic field and the other has a negative or south magnetic field. It is also found that the field is the strongest in the darker part of the spot, and weakest on the lighter part.

A further aspect of sunspots is that they are a source of significant levels of electromagnetic energy and cosmic rays. This in turn has a significant effect on the ionosphere and ionospheric radio propagation.

Sunspot numbers & Wolf number

In view of the fact that sunspots and the number of them has such a large impact on HF ionospheric radio propagation, it is useful to be able to quantify them and their affects.

It took many years after the first observations of sunspots for scientists to realise there was a cycle. This was because they were so random in nature and they also appeared and disappeared with the rotation of the Sun.

One of the first steps in quantifying sunspot activity was devised by Rudolf Wolf, director of the Zurich observatory. He devised a scheme which is still used today and it is called the Wolf Number.

The activity is defined in terms of a "sunspot number", which is not the actual number of sunspots which are seen.

The formula is given below:

Where:
      R = the Wolf number for sunspot activity
      k = a correction factor to take account of the equipment and observer characteristics
      g = the number of sunspot groups
      s = the number of observable spots (whether individually or in groups)

It might appear that the Wolf number is heavily weighted towards sunspot clusters. This was done because Wolf deduced that clusters of sunspots were a better indication of sunspot numbers that individual ones.

Although the Wolf number is far from perfect, it does have its uses and as measurements have been made using this formula for over 200 years, it gives a good standard for comparison.

Smoothed sunspot numbers

One of the reasons that any patterns in the sunspot numbers were not detected was that they varied so widely on a day to day basis

it was only when they were averaged or smoothed over a period of time, that any patterns became obvious.

A scheme called the "Smoothed Sunspot Number", SSN was introduced to add averaging to the numbers.

To generate the numbers, first the daily numbers are averaged over the period of a month. These numbers are then taken and smoothed over a 12 month period.

In order to ensure that the mean falls right in the middle of the month in question rather than between months, the period of smoothing is run over thirteen months, but half the value for the months at either end it taken so that effectively a 12 month period is averaged.

The formula is given below:

where:
      R_s = the smoother sunspot number
      R_m1 to R_m13 = the monthly averaged numbers for months 1 to 13

The figures for both the monthly and monthly average sunspot numbers and the smoothed values are used in propagation predictions are widely available. However the smoothed figures are obviously much in arrears because the figures for a particualr moth use figures for six months either side, and figures into the future are obviously not know!

The numbers are now prepared by the Sunspot Index Data Centre in Brussels from information supplied by a number of observatories.

It is found that the 12 month smoothed sunspot number correlates quite closely with the prevailing HF radio propagation conditions.

Sunspot cycle

When smoothed sunspot numbers are used, it can be clearly seen that there is a cyclical variation over a period of very approximately 11 years.

However, using all the figures back to the earliest data collected around 1755 and using smoothed values, it because obvious there was a cyclical variation of the number of sunspots.

Although everyone talks about an 11 year cycle, the actual period can vary quite considerably - it can be anywhere between about 7 and 17 years, although the average is 10.9 years.

The maximum sunspot activity numbers vary from 49 to 200 with an average of just over 100. The minimum number can be anywhere between none and 12.

In terms of the shape of the curve for any plots of sunspot numbers, it is usually found that after the sunspot minimum, sunspot activity rises sharply, reaching the peak in around four years, and after this it falls away more slowly taking around seven years to decay. Naturally this figure too varies very widely and it can only be taken as a very rough guide.

The sunspot cycle is of great interest to anyone using the HF portion of the radio spectrum. Propagation conditions are greatly influenced by sunspot activity, and accordingly they vary in line with the sunspot cycle. At the low point of the cycle, the high frequency bands above 20 MHz or so may not support ionospheric reflections, whereas at the peak of the cycle frequencies at 50 MHz and higher may be reflected.

The sunspot activity is of great importance to anyone involved in HF radio communications. Whether two way radio communications, maritime mobile communications, general mobile communications, point to point radio links, amateur radio communications, radio broadcasting or whatever form of radio communications. The level of sunspot activity has an enormous effect on the ionosphere and hence on HF radio propagation conditions. Accordingly even a superficial understanding is is essential using HF radio of almost any form.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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